Over 80% of smokers want to quit smoking and over half attempt to quit each year; however, less than 3% succeed in quitting. The PHS Clinical Practice Guideline [USPHS, 2000] reports the use of nicotine replacement therapy (NRT) approximately doubles quit rates and recommends use of NRT by all smokers who attempt to quit unless contraindicated. However, the effectiveness of NRT is limited in real-world practice by under-use of NRT by smokers attempting to quit and lack of adherence to dosing instructions. There is a clear need to reach more smokers with accessible behavioral treatment and support to increase the use of and adherence to NRT. The Internet is an ideal delivery channel for behavioral cessation treatment and medication support. Each year approximately 7 million Americans turn to the Internet for information and help quitting smoking. QuitNet.com is a field-tested website that provides cessation treatment in accordance with the PHS Guidelines and is interactive. The goal of this project is to improve the rate of utilization and adherence to NRT among smokers attempting to quit. In Phase I we developed a comprehensive Medication Support System (MSS) and tested it in focus groups with QuitNet subscribers. Feedback from the focus groups was very positive regarding its value and usability. In Phase II, we will complete integrative programming of the MSS system and conduct a randomized trial to test its efficacy. We will recruit 500 adult smokers through our partnership with Blue Cross Blue Shield of Rhode Island. Using a randomized, controlled design with repeated measures at baseline, 3-, 6-, and 12-months, smokers will be assigned to one of two treatments: 1) QuitNet (QN); or 2) QuitNet with the MSS (QN+MSS). We hypothesize that at month 12, participants in the QN+MSS group will have significantly higher quit rates compared to QN participants. We will also examine potential mediators of cessation including medication use and adherence as well as dose of behavioral treatment (e.g., number of logins to QuitNet). Lastly, we will examine normative patterns of medication use and adherence among participants in the control condition to establish a base- rate against which to compare the efficacy of QuitNet and QuitNet plus Medication Support. The proposed project could have significant public health impact by increasing the number of smokers who use NRT when quitting, and by improving quit rates through improved adherence with recommended NRT guidelines.